Metal complexes with 2-(α-hydroxy-benzyl) thiamin pyrophosphate (HBTPP). Models for metal binding of thiamin enzymes

The reactions of MCl₂ (M = Zn²⁺, Cd²⁺, Hg²⁺) with 2-(α-hydroxy-benzyl)thiamin pyrophosphate (HBTPP) at various pH values (different protonation states) were studied in methanolic solutions. Solid complexes of formulae K[Zn(HBTPP) ⁻Cl₂ · H₂O_n, K₂[Cd(HBTPP)²⁻Cl₂ · 3H₂O_n, K₂[Hg(HBTPP)₂⁻Cl₂ · 3H₂O and Zn(HBTPP)₂⁰Cl₂ were isolated and characterized by elemental analysis and various NMR techniques, namely ¹³C NMR, ³¹P NMR, ¹¹³Cd NMR, ¹⁹⁹Hg NMR and ¹H NMR ROESY spectra in D₂O. The data provide evidence that Zn(II) in K[Zn(HBTPP) ⁻Cl2 · H₂O_n, and Cd(II) in K₂[Cd(HBTPP)²⁻Cl₂ · 3H₂O_n, are coordinated both to the pyrimidine N(1′) and to the pyrophosphate group. In contrast, Hg(II) in K₂[Hg(HBTPP)₂⁻Cl₂ · 3H₂O and Zn(II) in Zn(HBTPP)₂⁰Cl₂ are bound only to the N(1′) atom or to the pyrophosphate group, respectively.     

Kinetic studies on the reaction of ethylenediaminetetraacetatochromium(III) complex with hydrogen peroxide

The rate of reaction of [Cr(III)Y]_aq (Y is EDTA anion) with hydrogen peroxide was studied in aqueous nitrate media [μ = 0.10 M (KNO₃)] at various temperatures. The general rate equation, Rate = $\text{k}{}_1\text{+}\text{k}{}_2\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}\text{1 +}\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}$ [Cr(III)Y]_aq[H₂O₂] holds over the pH range 5–9. The decomposition reaction of H₂O₂ is believed to proceed via two pathways where both the aquo and hydroxo-quinquedentate EDTA complexes are acting as the catalyst centres. Substitution-controlled mechanisms are suggested and the values of the second-order rate constants k₁ and k₂ were found to be 1.75 × 10^?2 M^?1 s^?1 and 0.174 M^?1 s^?1 at 303 K respectively, where k₂ is the rate constant for the aquo species and k₂ is that for the hydroxo complex. The respective activation enthalpies (ΔH^*₁ = 58.9 and ΔH^*₂ = 66.5 KJ mol^?1) and activation entropies (ΔS^*₁ = ?85 and ΔS^*₂ = ?40 J mol^?1 deg^?1) were calculated from a least-squares fit to the Eyring plot. The ionisation constant pK₁, was inferred from the kinetic data at 303 K to be 7.22. Beyond pH 9, the reaction is markedly retarded and ceases completely at pH ? 11. This inhibition was attributed in part to the continuous loss of the catalyst as a result of the simultaneous oxidation of Cr(III) to Cr(VI).     

The hydrogen bonding effect on the halobismuthate(III) geometry: thermal,spectroscopic and structural properties of halobismuthate complexes of 4,6-dimethylpyrimidine-2(1H)-thione

Reactions involving Bi(III) halides and 4,6- dimethylpyrimidine-2(1H)-thione (L) in HX solution result in the formation of [HL]₃[BiX₆]·2H₂O (X=C1, Br) and [HL]₃[Bi₂I₉]. These compounds together with the organic molecule in the form of the hydrochloride, (HLCl) were characterized by means of spectroscopic and thermogravimetric measurements. For HLCl·H₂O (1) and [HL]₃[BiCl₆]·2H₂O (2), X-ray structures were determined. In 1, which crystallizes in the space group Pca2₁, with four molecules in the cell, the structure consists of roughly planar protonated organic molecules stacked along the [100] axis and built up by hydrogen bonds involving chlorine atoms and water molecules. For 2, the space group is P2₁/n, Z=4, the structure contains [BiCl₆]³⁻ anions, protonated organic molecules stacked along the [010] axis and water molecules which form strong hydrogen bonds with the [BiCl₆]³⁻ anions. The final R indices are 0.0320 and 0.0465 for 1 and 2, respectively.     

The kinetics of the complex formation between iron(III)-ethylenediaminetetraacetate and hydrogen peroxide in aqueous solution

The kinetics of the formation of the purple complex [Fe^III(EDTA)O₂]³⁻, between Fe^III-EDTA and hydrogen peroxide was studied as a function of pH (8.22-11.44) and temperature (10-40 °C) in aqueous solutions using a stopped-flow method. The reaction was first-order with respect to both reactants. The observed second-order rate constants decrease with an increase in pH and appear to be related to deprotonation of Fe^III-EDTA ([Fe(EDTA)H₂O]⁻ ⇔ Fe(EDTA)OH]²⁻ + H⁺). The rate law for the formation of the complex was found to be d[Fe^IIIEDTAO₂]³⁻/dt=[(k₄[H⁺]/([H⁺] + K₁)][Fe^III-EDTA][H₂O₂], where k₄=8.15±0.05×10⁴ M⁻¹ s⁻¹ and pK₁=7.3. The steps involved in the formation of [Fe(EDTA)O₂]³⁻ are briefly discussed.     

Kinetics of reduction of ferrioxamine B by chromium(II), vanadium(II), and dithionite

Kinetic studies of the reduction of ferrioxamine B (Fe(Hdesf)⁺) by Cr(H₂O)₆²⁺, V(H₂O)₆²⁺, and dithionite have been performed. For Cr(H₂O)₆²⁺ and V(H₂O)₆²⁺, the rate is ?d[Fe(Hdesf)⁺]/dt = k[Fe(Hdesf)⁺][M²⁺]. For Cr(H₂O)₆²⁺, k = 1.19 × 10⁴ M^?1 sec^?1 at 25°C and μ = 0.4 M, and k is independent of pH from 2.6 to 3.5. For V(H₂O)₆²⁺, k = 6.30 × 10² M^?1 sec^?1 at 25°C, μ = 1.0 M, and pH = 2.2. The rate is nearly independent of pH from 2.2 to 4.0. For Cr(H₂O)₆²⁺ and V(H₂O)₆²⁺, the activation parameters are ΔH^≠ = 8.2 kcal mol^?1, ΔS^≠ ?12 eu and ΔH^≠ = 1.7 kcal mol^?1, ΔS^≠ = ?40 eu (at pH 2.2) respectively. Reduction by Cr(H₂O)₆²⁺ is inner-sphere, while reduction by V(H₂O)₆²⁺ is outer-sphere. Reduction by dithionite follows the rate law ?d[Fe(Hdesf)⁺]/dt =kK^{$\text{1}\text{2}$}[Fe(Hdesf)⁺][S₂O₄^2?]^{$\text{1}\text{2}$} where K is the equilibrium constant for dissociation of S₂O₄^2? into SO₂^? radicals. The value of k at 25°C and μ = 0.5 is 2.7 × 10³ M^?1 sec^?1 at pH 5.8, 3.5 × 10³ M^?1 sec^?1 at pH 6.8, and 4.6 × 10³ M^?1 sec^?1 at pH 7.8, and ΔH^≠ = 6.8 kcal mol^?1 and ΔS^≠ = ?19 eu at pH 7.8.     

Polyoxometalate/laccase-mediated oxidative polymerization of catechol for textile dyeing

The synergistic effect between polyoxometalates (POMs), namely K₅[SiW₁₁V^VO₄₀]·11H₂O and H₅[PMo₁₀V^V ₂O₄₀]·13H₂O and laccase from ascomycete Myceliophthora thermophila has been employed for the first time in oxidative polymerization of catechol. Such a laccase-mediator system allowed the formation of a relatively high molecular weight polycatechol as confirmed by size exclusion chromatography and electrospray ionization mass spectrometry (ESI-MS) (3990 Da when using K₅[SiW₁₁V^VO₄₀]·11H₂O and 3600 Da with H₅[PMo₁₀V^V ₂O₄₀]·13H₂O). The synthesized polymers were applied as dyes for the dyeing of flax fabrics. The color intensity of flax fabrics colored with polymer solutions was evaluated by diffuse reflectance spectrophotometry via k/s measurements (+10% of fixation ratio). A new synthetic process allowed a dyeing polymer, provided upon flax coloration, better color fixation and color resistance when compared to that obtained by conventional synthesis with laccase solely or with addition of organic mediator (1-hydroxybenzotriazole).     

Partitioning of taxifolin-iron ions complexes in octanol-water system

The composition of taxifolin-iron ions complexes in an octanol-water biphasic system was studied using the method of absorption spectrophotometry. It was found that at pH 5.0 in an aqueous biphasic system the complex of [Tf · Fe₂(OH) _k (H₂O)_{8 ? k} ] is present, but at pH 7.0 and 9.0 the complexes of [Tf₂ · Fe(OH) _k (H₂O)_{2 ? k} ] and [Tf · Fe(OH) _k (H₂O)_{4 ? k} ] are predominantly observed. The formation of a stable [Tf₃ · Fe] complex occurred in octanol phase. The charged iron ion of this complex is surrounded by taxifolin molecules, which shield the iron ion from lipophilic solvent. During transition from water to octanol phase the changes of the composition of complexes are accompanied by reciprocal changes in portion of taxifolin and iron ions in these phases. It was shown that the portion of taxifolin in aqueous solution in the presence of iron ions is increased at high pH values, and the portion of iron ions is minimal at pH 7.0. In addition, the parameters of solubility limits of taxifoliniron ions complexes in an aqueous solution were determined. The data obtained gain a better understanding of the role of complexation of polyphenol with metal of variable valency in passive transport of flavonoids and metal ions across lipid membranes.     

Coordination properties of α-hydroxy carboxylic acids. Part I. Binuclear niobium (V) complex acids and some salts

The conditions of dissolution of freshly precipitated niobium (V) oxide in α-hydroxy carboxylic acids glycolic, lactic, malic and tartaric were investigated. The dissolution is a function of the molar ratio α-hydroxy carboxylic acid/hydrated niobium(V) oxide, pH of the solution, temperature and time. From solutions of α-hydroxy monocarboxylic acids at 2 < pH < 3 the binuclear complexes H₃O[Nb₂O₄(C₂H₂O₃)(C₂H₃O₃)]·H₂O and H₃O[Nb₂O₄(C₃H₄O₃)(C₃H₅O₃)]·H₂O were isolated. Colourless, poorly-crystalline complexes are 1:1 electrolytes and, according to i.r. spectral evidence, the binuclearity in their structures is achieved through oxygen bridges. With α-hydroxy dicarboxylic acids crystalline M[Nb₂O₃(C₄H₃O₅)(C₄H₄O₅)]·nH₂O and poorly crystalline complexes, M₂[Nb₂O₂(C₄H₂O₆)₂]·nH₂O, M = H₃O⁺, NH₄⁺ were prepared as 1:1 electrolytes for the former and 1:2 electrolytes for the latter. Analytical, spectral, conductometric and potentiometric titration data give evidence for binuclear malatoniobate(V) and tartratoniobate(V) anions with bridging complex-forming agents.     

Non-enzymatic hydrolysis of creatine ethyl ester

The rate of the non-enzymatic hydrolysis of creatine ethyl ester (CEE) was studied at 37 °C over the pH range of 1.6-7.0 using ¹H NMR. The ester can be present in solution in three forms: the unprotonated form (CEE), the monoprotonated form (HCEE⁺), and the diprotonated form (H₂CEE²⁺). The values of pK_a1 and pK_a2 of H₂CEE²⁺ were found to be 2.30 and 5.25, respectively. The rate law is found to be

Rate=-dC_CEE/dt=k₊₊[H₂CEE²⁺][OH^-]+k₊[HCEE⁺][OH^-]+k₀[CEE][OH^-]     

Lower postprandial oxidative stress in women compared with men

Background: Previous studies indicate that oxidative stress is increased following intake of a high-fat meal, mediated in large part by the triglyceride (TG) response to feeding as well as fasting oxidative stress values. It has been suggested that women may process TG more efficiently after high-fat meals, based on the antilipidemic properties of estrogen. It has also been reported that women present with lower fasting oxidative stress values than do men. It is possible that women experience attenuated postprandial oxidative stress compared with men.Objective: The purpose of this study was to compare the postprandial TG and oxidative stress response after a lipid meal in healthy men and women.Methods: This study was conducted at The University of Memphis in Memphis, Tennessee, from October to December 2008. Blood samples were collected before (in a 10-hour fasted state), and at 1, 2, 4, and 6 hours after ingestion of a lipid load (heavy whipping cream at 1 g · kg^?1). Blood samples were analyzed for TG, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and nitrate/nitrite (NOx). The AUC was calculated for each variable and results were compared using a t test. Effect-size calculations were performed using Cohen's d.Results: Samples from 10 men and 10 women, aged 18 to 47 years (17 subjects aged <37 years), were compared. AUC data were not significantly different for TG (mean [SEM] 330 [48] vs 354 [34] mg · dL^?1 · 6h^?1 for men and women, respectively; effect size = 0.09) or NOx (165 [25] vs 152 [17] μmol · L^?1 · 6h^?1 for men and women; effect size = 0.09). However, significant differences were noted for MDA (10.7 [1.3] vs 6.1 [0.5] μmol · L^?1 · 6h^?1 for men and women, respectively; P = 0.002; effect size = 0.61) and H₂O₂ (154 [23] vs 86 [8] μmol · L^?1 · 6h^?1 for men and women; P = 0.013; effect size = 0.53).Conclusions: These data indicate that women experience lower oxidative stress than do men, with regard to MDA and H₂O₂, after ingestion of a lipid load in the form of heavy whipping cream. Considering the strong association between oxidative stress and cardiovascular disease, lower postprandial oxidative stress may be one mechanism associated with decreased risk of cardiovascular disease in women compared with men. Further research is needed to confirm this hypothesis.     

Reversible linkage isomerization of pentaamminecobalt(III) complexes of urea and its N-methyl derivatives

The (NH₃)₅CoOC(NH₂)₂³⁺ ion is consumed in water according to the rate law k(obs.) = k₁ + k₂[OH⁻], where k₁ = 4.0 × 10⁻⁵ s⁻¹ and k₂ = 14.2 M⁻¹ s⁻¹ (0–0.1 M [OH⁻];μ = 1.1 M, NaClO₄, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH₃)₅CoOH₂³⁺ is observed, but in 0.1–1.0 M [OH⁻], 7% of the directly formed products is the urea-N complex (NH₃)₅CoNHCONH₂²⁺ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea-N species. These results are interpreted in terms of an urea-O to urea-N linkage isomerization reaction competing with hydrolysis for both spontaneous (k₁) and base-catalyzed (k₂) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (pK′_a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k₂[OH⁻]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k₁ + k₂[OH⁻] = (k_s + k_ON) + (k_OH + k′_ON) [OH⁻] for the reactions of the O-bonded isomers, where k_s, k_OH are the specific rates for hydrolysis, and k_ON, k′_ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; k_ON = 1.3 × 10⁻⁵ s⁻¹ and k′_ON = 1.1 M⁻¹ s⁻¹ (μ = 1.0 M, NaClO₄, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N,N-dimethylurea and N-phenylurea, but not for the N,N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH₂ capture (versus H₂O), while −NHR and −NR₂ do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea-O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H₂O, Me₂SO) is discussed. The urea-N complexes have been isolated as their deprotonated forms, [(NH₃)₅CoNHCONRR′](ClO₄)₂·xH₂O (R,R′ = H, CH₃). They are kinetically inert in neutral to basic solution but in acid they protonate (H₂O, pK′_a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H₂O and Me₂SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form k_NO(obs.) = (k + k_NO)[H⁺]/(K′_a + [H⁺]) and the active species in the reaction is the protonated form; k, k_NO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me₂SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH₃)₅CoNH₂CONH₂³⁺, diastereotopic exo-NH₂ protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. ¹³C and ¹H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea-N/urea-O complex equilibrium is governed by the relation K′_NO(obs.) = K′_NO[H⁺]/[H⁺](K′_a), where K′_NO is the equilibrium constant = [(NH₃₅Co(urea-O)³⁺]/[(NH₃)₅Co(urea-N)³⁺]. Values for K′_NO(=k_NO/k_ON = 260 and pK′_a ≈ 3 for the NH₂CONH₂ system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′_NO(obs.) = 2.6 × 10⁻²) and instability in acid solution (e.g. pH 1, K′_NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH₃)₅Co(III) are contrasted with the result for the analogous Rh(III)NH₂CONH₂ system K′_NO ≈ 1).     

Oxoperoxofluoromolybdate(VI) complexes. The first synthesis of oxodiperoxofluoromolybdate(VI) complexes

A number of oxoperoxofluoromolybdate(VI) complexes, viz. M₂[MoO(O₂)F₄] and M₂[MoO(O₂)₂F₂] where M = K and NH₄ and K[MoO(O₂)₂F]·2H₂O have been synthesised by various methods. The infrared spectra suggest that the peroxide groups are bonded to the molybdenum centre in a triangular bidentate manner.     

Equilibrium constants under physiological conditions for the reactions of L-phosphoserine phosphatase and pyrophosphate: L-serine phosphotransferase

The observed equilibrium constants (K_obs) for the l-phosphoserine phosphatase reaction [EC 3.1.3.3] have been determined under physiological conditions of temperature (38 °C) and ionic strength (0.25 m) and physiological ranges of pH and free [Mg²⁺]. Using Σ and square brackets to indicate total concentrations $\text{K}{}_{\mathrm{<mtext>obs</mtext>}}\text{=}\text{Σ L-serine][Σ P}{}_{\mathrm{i}}\text{]}\text{Σ L-phosphoserine]H}{}_2\text{O]}\text{, K =}\text{L-H · serine}{}^{\mathrm{\pm }}\text{]HPO}{}_4{}^{\mathrm{2?}}\text{]}\text{[L-H · phosphoserine}{}^{\mathrm{2?}}\text{]H}{}_2\text{O]}$. The value of K_obs has been found to be relatively sensitive to pH. At 38 °C, K⁺] = 0.2 m and free [Mg²⁺] = 0; K_obs = 80.6 m at pH 6.5, 52.7 m at pH 7.0 [ΔG_obs⁰ = ?10.2 kJ/mol (?2.45 kcal/mol)], and 44.0 m at pH 8.0 ([H₂O] = 1). The effect of the free [Mg²⁺] on K_obs was relatively slight; at pH 7.0 ([K⁺] = 0.2 m) K_obs = 52.0 m at free [Mg²⁺] = 10^?3, m and 47.8 m at free [Mg²⁺] = 10^?2, m. K_obs was insignificantly affected by variations in ionic strength (0.12–1.0 m) or temperature (4–43 °C) at pH 7.0. The value of K at 38 °C and I = 0.25 m has been calculated to be 34.2 ± 0.5 m [ΔG_obs⁰ = ?9.12 kJ/mol (?2.18 kcal/ mol)]([H₂O] = 1). The K for the phosphoserine phosphatase reaction has been combined with the K for the reaction of inorganic pyrophosphatase [EC 3.6.1.1] previously estimated under the same physiological conditions to calculate a value of 2.04 × 10⁴, m [ΔG_obs⁰ = ?28.0 kJ/mol (?6.69 kcal/mol)] for the K of the pyrophosphate:l-serine phosphotransferase [EC 2.7.1.80] reaction. $\text{K}{}_{\mathrm{<mtext>obs</mtext>}}\text{=}\text{[}\text{Σ L-serine}\text{][}\text{Σ}\text{P}{}_{\mathrm{<mtext>i</mtext>}}\text{]}\text{[}\text{Σ L-phosphoserine}\text{][}\text{H}{}_2\text{O}\text{]}\text{, K =}\text{[L-H · serine}{}^{\mathrm{\pm }}\text{]HPO}{}_4{}^{\mathrm{2?}}\text{]}\text{[L-H · phosphoserine}{}^{\mathrm{2?}}\text{]H}{}_2\text{O}$. Values of K_obs for this reaction at 38 °C, pH 7.0, and I = 0.25 m are very sensitive to the free [Mg²⁺], being calculated to be 668 [ΔG_obs⁰ = ?16.8 kJ/mol (?4.02 kcal/mol)] at free [Mg²⁺] = 0; 111 [ΔG_obs⁰ = ?12.2 kJ/mol (?2.91 kcal/mol)] at free [Mg²⁺] = 10^?3, m; and 9.1 [ΔG_obs⁰ = ?5.7 kJ/mol (?1.4 kcal/mol) at free [Mg²⁺] = 10^?2, m). K_obs for this reaction is also sensitive to pH. At pH 8.0 the corresponding values of K_obs are 4000 [ΔG_obs⁰ = ?21.4 kJ/mol (?5.12 kcal/mol)] at free [Mg²⁺] = 0; and 97.4 [ΔG_obs⁰ = ?11.8 kJ/ mol (?2.83 kcal/mol)] at free [Mg²⁺] = 10^?3, m. Combining K_obs for the l-phosphoserine phosphatase reaction with K_obs for the reactions of d-3-phosphoglycerate dehydrogenase [EC 1.1.1.95] and l-phosphoserine aminotransferase [EC 2.6.1.52] previously determined under the same physiological conditions has allowed the calculation of K_obs for the overall biosynthesis of l-serine from d-3-phosphoglycerate. $\text{K}{}_{\mathrm{<mtext>obs</mtext>}}\text{=}\text{[}\text{Σ L-serine}\text{][}\text{Σ NADH}\text{][}\text{Σ}\text{P}{}_{\mathrm{<mtext>i</mtext>}}\text{][}\text{Σ}\text{α-}\text{ketoglutarate}\text{]}\text{[Σ d-3-}\text{phosphoglycerate}\text{][Σ NAD}{}^{\mathrm{+}}\text{][Σ L-glutamat0]}$ The value of K_obs for these combined reactions at 38 °C, pH 7.0, and I = 0.25 m (K⁺ as the monovalent cation) is 1.34 × 10^?2, m at free [Mg²⁺] = 0 and 1.27 × 10^?2, m at free [Mg²⁺] = 10^?3, m.     

Homogeneous oxidative coupling catalysts. Mechanism of conversion of 2,6-dimethylphenol [DMP] to 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone [DPQ] by [(Pip)nCuX]4O2 (n = 1 or 2, Pip = piperidine, X = Cl, Br or I) in aprotic media

This paper represents the mechanism of the second half of the catalytic cycle, Scheme 1, which represents the conversion of 2,6-dimethylphenol [DMP] to 3,3′,5,5′-tetramethyl, 4,4′-diphenoquinone [DPQ] by homogenous oxidative coupling catalysts [(Pip)_nCuX]₄O₂ in aprotic media. The mechanism can be represented as a pre-equilibrium, K, between the catalyst and 2,6-dimethylphenol to form a complex intermediate which is converted into the activated complex through the rate determining step, k₂, to form the final products. The observed pseudo first-order rate constant is given by k_obs = K k₂[DMP]^y/(1 + K[DMP]^y). When the coordination number around copper(II) is equal to five as in [(Pip)CuX]₄O₂, the system suffers from kinetic saturation due to strong complex formation between catalyst and [DMP] and therefore K[DMP]^y > 10 and k_obs = k₂. Kinetic saturation has been avoided by using six coordinate copper(II) as in [(Pip)₂CuX]₄O₂. The influence of the coordination saturation of copper(II) in [(Pip)₂CuX]₄O₂ helps to evaluate both thermodynamic and kinetic parameters for the system as well as for the structure of the activated complex, (y = 2), which consists of one [(Pip)₂CuX]₄O₂ and two [DMP]. Reduction of copper(II) to copper(I) has been suggested as a rate determining step due to halogen, X, and solvent effects.     

Pulse radiolytic study of α-tocopherol radical mechanisms in ethanolic solution

Pulse radiolytic studies of α-tocopherol (αTH) oxidation-reduction processes were carried out with low doses (5 Gy) of high-energy electrons in O₂₋, N₂₋, and air-saturated ethanolic solutions. Depending on the concentration of oxygen in solution, two different radicals, A· and B·, were observed. The first, A·, was obtained under N₂ and results from aTH reaction with solvated electron (k_aTH+csolv⁻ = 3.4 × 10⁸ mol⁻¹ liter s⁻¹) and with H₃C-ĊH-OH, (R·) (k_{aTH + R·} = 5 × 10⁵ mol⁻¹ liter s⁻¹). B·, observed under O₂, is produced by αTH reaction with RO₂ peroxyl radicals (k_aTH + RO₂^. = 9.5 × 10⁴ mol⁻¹ liter s⁻¹).     

Synthesis,crystal structures and properties of three rare earth substituted germanotungstates: M/[α-GeW11O39] (M = Nd,Eu, and Tb)

Three new polyoxometalate compounds based on the lacunary Keggin anion [α-GeW₁₁O₃₉]^8? and the rare earth cations (Ln = Nd^III, Eu^III, Tb^III), [(CH₃)₄N]_2.5H_7.5[Eu(GeW₁₁O₃₉)(H₂O)₂]₂ · 4.5H₂O (1), [(CH₃)₄N]₂H₈[Tb(GeW₁₁O₃₉)(H₂O)₂]₂ · 2.5H₂O (2) and [Nd_0.5(H₂O)₂]H_0.5[Nd₂(GeW₁₁O₃₉)(DMSO)₂(H₂O)₈] · 5.5H₂O (3), have been synthesized and characterized by elemental analysis, inductively coupled plasmas (ICP) analysis, IR spectroscopy, single-crystal X-ray diffraction. The solid-state structures of compounds 1 and 2 consist of one-dimensional linear wires built of [α-GeW₁₁O₃₉]^8? anions connected by Eu³⁺/Tb³⁺ cations, while in compound 3, the introduction of the organic molecules DMSO (DMSO = dimethyl sulphoxide) leads to a double-parallel chainlike structure constructed by two linear wires {[Nd(1)(GeW₁₁O₃₉)(DMSO)(H₂O)₂]^5?}_n linked by Nd³⁺ coordination cation. Furthermore, the luminescent property of compound 1 and the thermal stability of compound 3 were also studied.     

Coordination chemistry of lanthanides with cryptands. An X-ray and spectroscopic study of the complex Nd2(NO3)6 [C18H36O6N2]·H2O

By reacting neodymium nitrate hexahydrate with the cryptand 〈222〉 in methanol, the complex Nd₂-(NO₃)₆[C₁₈H₃₆O₆N₂]·H₂O was obtained and analyzed by single-crystal X-ray diffraction. The cell is triclinic P$\text{1}$ with a = 14.870(2) Å, b = 13.261(2) Å, c = 8.832(1) Å, α = 91.2(1)°, β = 93.4(1)°, γ = 87.6(1)°, Z = 2 and U = 1736.6 ų. The structure was refined by least-squares methods to the conventional R = 0.039 for 6177 observed reflections. The compound contains the cations [Nd〈222〉(NO₃)]²⁺ and the anions [Nd(NO₃)₅·H₂O]^2?, and is isostructural with the samarium analogue. Solid state fluorescence spectra of the title complex were measured at room and liquid nitrogen temperature, and the transitions ⁴F_3/2 → ⁴I_9/2 and ⁴F_3/2 → ⁴I_11/2 analyzed.     

A reaction of the superoxide radical with tetrapyrroles

Bilirubin and biliverdin were bleached during exposure to the aerobic xanthine oxidase reaction. Enzymic scavenging of O₂^?, by Superoxide dismutase, inhibited, whereas enzymic scavenging of H₂O₂, by catalase, did not. Increasing the rate of production of O₂^? without increasing the turnover rate of xanthine oxidase, by increasing pO₂, accelerated the bleaching of the biliverdin. Moreover, a scavenger of OH·, such as benzoate, or an inactivating chelating agent for iron, such as diethylenetriamine pentaacetate or desferrioxamine mesylate, did not inhibit. It follows that O₂^? can directly attack these tetrapyrroles. Kinetic competition between Superoxide dismutase and bilirubin yielded a value for k_{bilirubin, O2^?} = 2.3 × 10⁴ M^?1s^?1 at pH 8.3 and at 23 °C. A similar experiment for biliverdin yielded a value for k_{bilirubin, O2^?} = 7 × 10⁴ M^?1s^?1.     

Kinetics of acid-catalysed dissociation of lead(II) and copper(II) complexes of ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA)

The acid-catalysed dissociation rate constants for PbEGTA²⁻ and CuEGTA²⁻ complexes (where EGTA is ethylenebis(oxyethylenenitrilo) tetraacetic acid) were measured in acetic acid-acetate buffer medium (pH: 3.0–4.8) and perchloric acid solutions ([H⁺] = 0.05–0.15 M), respectively, at a constant ionic strength of 0.15 (NaClO₄). The rate laws shown by the lead(II) and copper(II) complexes are of the form, Rate = {k_d + k_H[H⁺]}[complex] and Rate = {k_d + k_H²[H⁺]²}[complex], respectively. Enthalpy and entropy of activation for acid-independent and acid-catalysed pathways for both the complexes were obtained by the temperature-dependence studies of resolved rate constants in the 16–45°C range. The rate of dissociation of PbEGTA²⁻ is not enhanced by increasing the concentration of acetate ion in the buffer, and the amount of total electrolyte in the reaction mixture has no pronounced effect on the dissociation rates of their the lead(II) or copper(II) complex. Attempts to study the kinetics of stepwise ligand unwrapping in the binuclear Cu₂EGTA complex were unsuccessful due to the extremely rapid dissociation of this complex to yield mononuclear CuEGTA²⁻.     

Suicide-Peroxide inactivation of microperoxidase-11: A kinetic study

The kinetics of microperoxidase-11 (MP-11) in the oxidation reaction of guaiacol (AH) by hydrogen peroxide was studied, taking into account the inactivation of enzyme during reaction by its suicide substrate, H₂O₂. Concentrations of substrates were so selected that: 1) the reaction was first-order in relation to benign substrate, AH and 2) high ratio of suicide substrate to the benign substrate, [H₂O₂]>>[AH]. Validation and reliability of the obtained kinetic equations were evaluated in various nonlinear and linear forms. Fitting of experimental data into the obtained integrated equation showed a close match between the kinetic model and the experimental results. Indeed, a similar mechanism to horseradish peroxidase was found for the suicide-peroxide inactivation of MP-11. Kinetic parameters of inactivation including the intact activity of MP-11, α_i, and the apparent inactivation rate constant, k_i, were obtained as 0.282 ± 0.006 min^{? 1} and 0.497 ± 0.013 min^{? 1} at [H₂O₂] = 1.0 mM, 27°C, phosphate buffer 5.0 mM, pH = 7.0. Results showed that inactivation of microperoxidase as a peroxidase model enzyme can occur even at low concentrations of hydrogen peroxide (0.4 mM).